Method for preparation of perfluoroalcanecarboxylic and perfluoroalkanesulfonic acids

ABSTRACT

The invention consists of a new method for preparing perfluoroalcanecarboxylic and perfluoroalcanesulfonic acids in which ozone is made to react, in a protic environment, upon a perfluoroalkyl chain comprising at least one oxydizable chemical group.

The invention relates to a novel process for the preparation ofperfluoroalkanecarboxylic acids and perfluoroalkanesulfonic acids.

Perfluoroalkanecarboxylic acids and perfluoroalkanesulfonic acids playan important role in the chemical industry of today ("Organof luorineChemistry, Principles and Commercial Applications", R. E. Banks, B. E.Smart, J. C. Tatlow, Eds., Plenum Press, New York 1994).

Perfluoroalkanecarboxylic acids and perfluoroalkanesulfonic acids find awide variety of uses in the chemical industry, both as syntheticintermediates and as finished products.

Long-chain perfluoroalkanecarboxylic acids and their salts are goodsurfactants which are able to reduce considerably the surface tension ofwater, of aqueous solutions and of organic liquids, even in lowconcentrations. These acids and their derivatives can also be used asemulsifiers, dispersing agents and foaming agents.

Short-chain perfluoroalkanesulfonic acids are used as intermediates inthe preparation of sulfonamides. The latter are employed as plant growthregulators and as herbicides.

Perfluoroalkanesulfonic acids are also used as antistatic andantisoiling agents for porous substrates such as paper and textiles.

Methods for preparing perfluroalkanecarboxylic acids andperfluoroalkanesulfonic acids are already known. Thus, these acids aregenerally prepared either by electrochemical fluorination or by the useof chemical agents with strong oxidizing power. However, these methodseach have their drawbacks.

The electrochemical fluorination technique is intricate and expensivebecause of the hazards associated with the use of liquid hydrogenfluoride and because of the corrosion of the electrolysis cells causedby the medium.

The oxidation routes using chemical agents comprise the action of hotoleum on perfluoroalkyl iodides. The use of oleum (very concentratedsulfuric acid containing sulfur trioxide, SO₃) is also hazardous onaccount of its corrosive properties. Furthermore, this method results inpartial degradation of the carbon chain, leading to the production of amixture of homologous fluoro acids.

The process according to the present invention uses ozone for thepreparation of perfluoroalkanecarboxylic acids andperfluoroalkanesulfonic acids. Ozone allows the starting materials to beoxidized mildly and selectively, thus overcoming the drawbacks andhazards associated with the methods of the prior art.

The subject of the present invention is thus a process for thepreparation of perfluoroalkanecarboxylic acids andperfluoroalkanesulfonic acids, characterized in that ozone is reacted,in a protic medium, with a perfluoroalkyl chain containing at least oneoxidizable chemical group.

The perfluoroalkyl chains which thus constitute the starting materialsin the context of the present invention can contain 1 to 12 carbon atomsand have the general formula

    CF.sub.3 (CF.sub.2).sub.n Y

in which Y represents the oxidizable chemical group, which can be chosenfrom:

compounds of formula CF₂ X, X being a hetero atom, preferably a halogenand even more preferably iodine, or

a heteroatomic group, preferably a sulfur-containing group, or

an aromatic group.

According to a preferred embodiment, the sulfur-containing group ischosen from aryl thioethers, the aryl group being chosen from mono- orbicyclic radicals which can contain at least one substituent chosen fromhydrogen, linear or branched alkyl radicals and halogen, ether, alkoxy,aryloxy, metal carboxylate, acyloxy, fluoroalkylthio, fluoroalkoxy andcarboxylic acid radicals.

According to another preferred embodiment of the invention, the aromaticgroup is a mono- or bicyclic aromatic radical which can contain at leastone substituent chosen from hydrogen, linear or branched alkyl radicalsand halogen, ether, alkoxy, aryloxy, metal carboxylate, acyloxy,fluoroalkyl, fluoroalkoxy and carboxylic acid radicals.

In accordance with the process according to the invention, the reactionsare carried out by simply placing the starting materials in contact withvapors containing ozone, which constitutes the most powerful oxidizingchemical element after fluorine, its standard oxidation potentialrelative to the standard hydrogen electrode being 2.07 V.

The reaction according to the present invention is carried out in proticmedium. According to an advantageous embodiment, the protic mediumconsists of water, of alcohol or of carboxylic acids, which preferablydo not contain fluorine, or mixtures thereof.

The reaction temperature is between -100° C. and 100° C., preferablybetween 0 and 40° C. Even more preferably, the reaction is carried outat a temperature close to room temperature.

As regards the pressure conditions, the reaction is generally carriedout at a pressure below 10 bar, more preferably at a pressure close toatmospheric pressure.

In general, the process according to the invention can thus be carriedout, with good efficacy under mild conditions, at room temperature andat atmospheric pressure.

According to an advantageous embodiment, the reaction according to thepresent invention can also be carried out in the presence of aninsoluble solid product with a large specific surface area, such as, forexample, silica, alumina, silica-alumina, titanium oxide, activecharcoal, peats, clays and zeolites. This allows the interaction betweenthe ozone and the perfluoro compounds to be enhanced considerably.

Given that the starting materials of formula CF₃ (CF₂)_(n) Y are madevery electron-poor by the fluorine atoms, it is extremely surprising andunexpected that this oxidation can take place at a temperature close toroom temperature, without any particular activation.

As regards the sulfur-containing starting materials, the prior artteaches that they can only be oxidized into the corresponding sulfoneswith the aid of very strong reagents, such as hot chromium oxide (R. M.Scribner, J. Org. Chem, 1966, 31 3671). This conversion has no effect onthe aromatic ring and does not produce polyfluoroalkanesulfonic acids.Moreover, common sulfides are oxidized with ozone into sulfoxides andthen into sulfones. Thus, methyl phenyl sulfide is converted into thecorresponding sulfone in quantitative yield (D. Barnard, J. Chem. Soc.,1957, 4547). It is thus particularly surprising that the oxidation withozone, according to the invention, of arylperfluoroalkyl sulfidesproceeds through to the formation of perfluoroalkanesulfonic acids,although their sulfur atom is less oxidizable than that of methyl phenylsulfide.

Without this theory being possibly construed as limiting, it is possiblethat the ozone first attacks the aromatic ring rather than the sulfuratom in the starting materials comprising a sulfur-containing group.

According to a preferred embodiment of the invention, the ozone/startingmaterial molar ratio is between 1 and 20, preferably between 2 and 5,for the perfluoroalkanecarboxylic acids, and between 3 and 8 for theperfluoroalkanesulfonic acids.

A cosolvent can advantageously be added in order to dissolve thestarting material. This cosolvent can be chosen from aprotic solventssuch as nitrites, in particular acetonitrile, organochlorine compounds,in particular tetrachloromethane, and secondary or tertiary amines, inparticular diethylamine, triethylamine and tributylamine and amineshaving one or more longer alkyl chains.

The duration of the reaction according to the invention can rangeapproximately between thirty minutes and seven days.

The aim of the examples which follow is to illustrate the invention moreclearly.

EXAMPLE 1

20 g of perfluorooctyl iodide (C₈ F₁₇ I) and 150 ml of propanoic acidare introduced into a glass reactor. The mixture is stirred at roomtemperature and the atmosphere of the container is flushed with anozone/oxygen mixture (6/94 volume/volume) for 2 days. The precipitateformed is then filtered off. The perfluorooctanoic acid (C₇ F₁₅ COOH)contained in the filtrate is precipitated with pentane, filtered off ona Buchner funnel and dried to give 5 g of a white solid. The yield forobtaining the perfluorooctanoic acid is thus 30%. This yield wascalculated after ¹⁹ F NMR analysis in CDCl₃ at 300 MHz, which gives thefollowing results:

δ (ppm) -80.9(3F, CF₃); -119.4(t, 2F,CF₂); -121.8(2F, CF₂);-122.2(2F,CF₂); -122.9(s, 2F, CF₂); -126.3(d, 2F, CF₂ CF₃).

EXAMPLE 2

The process is performed as in Example 1. After reaction for twelvehours at room temperature, analysis of the residue indicates theformation of perfluorohexanoic acid (C₅ F₁₁ COOH) in a yield of 25%.This yield was calculated after ¹⁹ F NMR analysis in CDCl₃ at 282 MHz,which gives the following results:

δ (ppm) -80.8(t, 3F, CF₃); -116.5(2F, CF₂); -122.1(qd, 4F, 2CF₂);-125.7(t, 2F, CF₂ CF₃).

EXAMPLE 3

1 g, i.e. 2.52 mmol, of tridecafluorohexylbenzene (PhC₆ F₁₃) is mixedwith 20 ml of methanol in a glass tube. The mixture is stirred at roomtemperature and an ozone/oxygen mixture (6/94 volume/volume) is appliedto the solution for 12 hours. Analysis of the mixture indicates theformation of perfluoroheptanoic acid (C₆ F₁₃ COOH).

¹⁹ F NMR analysis in CDCl₃ at 282 MHz gave the following results:

δ (ppm) -80.54 (t, 3F, CF₃); -116.2(d, 2F, CF₂ COOH); -121.16(d, 2F,CF₂); -121.9(d, 2F, CF₂); -122.15(d, 2F, CF₂); -125.6(2F, CF₂ CF₃).

EXAMPLE 4

0.95 g of perfluorobutyl phenyl sulfide (2.89 mmol) is mixed with 20 mlof methanol and one equivalent of triethylamine, i.e. 0.4 ml, is added.The ozone/oxygen mixture (6/94 volume/volume) is bubbled into the liquidfor 7 hours. Next, a second equivalent of amine is added and theozonolysis is continued for a further 7 hours at room temperature. A50/50 mixture of the carboxylic acid (C₃ F₇ COOH) and of the sulfonicacid (C₄ F₉ SO₃ H) is obtained.

¹⁹ F NMR analysis in CDCl₃ at 282 MHz gave the following results:

C₄ F₉ SO₃ H

δ (ppm) -80.6(3F, CF₃); -114.1(2F, CF₂ SO₃); -120.9(2F, CF₂); -125.4(2F,CF₂ CF₃).

C₃ F₇ CO₂ H

δ (ppm) -80.6(3F, CF₃); -117.0(2F, CF₂ COOH); -125.5(2F, CF₂ CF₃).

EXAMPLE 5

1 g, i.e. 5.6 mmol, of phenyl trifluoromethyl sulfide is dissolved in 20ml of a methanol/water mixture (8/2 volume/volume). The ozone/oxygenmixture (6/94 volume/volume) is bubbled into the reaction liquid for sixhours.

Analysis of the mixture indicates the formation of methyltrifluoromethanesulfonate (CF₃ SO₃ Me) and of the corresponding acid ina 3/1 ratio.

Hydrolysis of the ester is carried out in the presence of baryta fortwelve hours. The excess baryta is neutralized and precipitated byadding dilute sulfuric acid until the pH is slightly acidic. Theresulting trifluoromethanesulfonic acid is then precipitated usingbarium carbonate.

The solvents are then evaporated off and the solid is extractedcontinuously with acetone for six hours.

¹⁹ F NMR analysis in CD₃ OD at 282 MHz gave the following results:

CF₃ SO₃ H δ (ppm) -78.2(3F, CF₃)

CF₃ SO₂ Me δ (ppm) -74.63(3F, CF₃)

(CF₃ SO₃)₂ Ba δ (ppm) -78.35(3F, CF₃)

EXAMPLE 6

1 g, i.e. 5.6 mmol, of trifluoromethyl phenyl sulfide (PhSCF₃) isdissolved in 20 ml of methanol. A spatula-tip amount (5 to 10 mg) oftitanium dioxide is added to the reaction mixture. The ozone-oxygenmixture (6/94 volume/volume) is bubbled into the reaction liquid for 4hours.

The relative percentages of the products formed are evaluated byfluorine NMR. Thus, the methyl ester of trifluoromethane sulfonic acid(CF₃ SO₃ H) is present in a proportion of about 15%, and thetrifluoromethanesulfonic acid is present in a proportion of about 15%.

EXAMPLE 7

1 g, i.e. 5.6 mmol, of trifluoromethyl phenyl sulfide (PhSCF₃) isdissolved in 20 ml of a methanolwater mixture (9/1 volume/volume). Aspatula-tip amount of silica is added to the reaction mixture. Theozone/oxygen mixture (6/94 volume/volume) is bubbled into he reactionliquid for 5 h 30.

The relative percentages of the products formed re evaluated by fluorineNMR. Thus, the methyl ester of rifluoromethanesulfonic acid is presentin a proportion of about 60% and trifluoromethanesulfonic acid ispresent in a proportion of about 30%.

What is claimed is:
 1. Process for the preparation of perfluoroalkanecarboxylic acids and perfluoroalkanesulfonic acids, according to which ozone is reacted, in a protic medium, with a perfluoroalkyl chain containing at least one oxidizable chemical group selected from the group consisting of a group of formula CF₂ X, X being a hetero atom, a heteroatomic group or an aromatic group.
 2. Process according to claim 1, wherein in the group of formula CF₂ X, X is a halogen atom.
 3. Process according to claim 2, wherein X is iodine.
 4. Process according to claim 1, wherein the hetero-atomic group is a sulfur-containing group except for a disulfide group.
 5. Process according to claim 4, wherein the sulfur-containing of group is selected from the group consisting of arylthioethers, the aryl group being chosen from mono- or bicyclic radicals which can contain at least one substituent chosen from hydrogen, linear or branched alkyl radicals and halogen, ether, alkoxy, aryloxy, metal carboxylate, acyloxy, fluoroalkylthio, fluoroalkoxy and carboxylic acid radicals.
 6. Process according to claim 1, wherein the aromatic group is a mono- or bicyclic aromatic radical which can contain at least one substituent selected from the group consisting of hydrogen, linear or branched alkyl radicals and halogen, ether, alkoxy, aryloxy, metal carboxylate, acyloxy, fluoroalkyl, fluoroalkoxy and carboxylic acid radicals.
 7. Process according to claim 1, wherein the protic medium is selected from the group consisting of water, alcohol, carboxylic acids, and mixtures thereof.
 8. Process according to claim 7, wherein the carboxylic acids do not contain fluorine.
 9. Process according to claim 1 wherein an aprotic cosolvent selected from the group consisting of nitriles, organochlorine compounds and secondary or tertiary amines, is added to the medium.
 10. Process according to claim 9, wherein the aprotic cosolvent is selected from the group consisting of acetonitrile, carbon tetrachloride, diethylamine, triethylamine and tributylamine.
 11. Process according to claim 1, wherein the temperature is between -100° C. and 100° C.
 12. Process according to claim 11, wherein the temperature is between 0 and 40° C.
 13. Process according to claim 12, wherein the temperature is close to room temperature.
 14. Process according to claim 1, wherein the pressure is less than 10 bar.
 15. Process according to claim 14, wherein the pressure is close to atmospheric pressure.
 16. Process according to claim 1, wherein the ozone/substrate molar ratio is between 1 and 20 for the perfluoroalkanecarboxylic acids, and between 3 and 8 for the perfluoroalkanesulfonic acids.
 17. Process according to claim 16, wherein the ozone/substrate molar ratio is between 2 and 5 for the perfluoroalkanecarboxylic acids.
 18. Process according to claim 1, wherein an insoluble solid product selected from the group consisting of silica, alumina, silica-alumina, titanium oxide, active charcoal, peats, clays and zeolites is added. 